The Skeletal System


I. Introduction


The skeletal system includes connective tissues such as blood, bone, cartilage, tendons, and ligaments. These tissues also combine with the various types of muscle tissue.


1) Bone itself has several functions, including:


A.    Mechanical

Support. Bones provide a framework for the attachment of

muscles and other tissues.

Movement. Bones enable body movements by acting as levers

and points of attachment for muscles.

B.    Protective

Bones such as the skull and rib cage protect vital organs

from injury. Bones also protect the marrow.

C. Metabolic

    Mineral storage. Bones serve as a reservoir for calcium and

    phosphorus, essential minerals for various cellular   

    activities throughout the body.
    Blood cell production. The production of blood cells, or

    hematopoiesis, occurs in the red marrow found within the

    cavities of certain bones.

    Energy storage. Lipids (fats) stored in adipose cells of the

    yellow marrow serve as an energy reservoir.


2) Cartilage serves as the fetal template for bone formation, and covers

    the ends of bone, most especially at the joints, or points of


CARTILAGE is an important part of the skeleton. What type of cartilage is the most common type?  HYALINE CARTILAGE makes up most of the EMBRYONIC SKELETON, but eventually is replaced by bone during fetal and childhood development. HYALINE CARTILAGE is also found at the ends of long bones at joints, connects the ribs to the breastbone, and forms the end of the nose. ELASTIC CARTILAGE gives shape to the outer ear.  FIBROCARTILAGE forms the intervertebral discs, between the vertebrae.

CARTILAGE resists compression (pushing forces) & tension (pulling forces) due to its rubbery ground substance (chondroitin sulfate) and collagen. Cartilage is also very resilient, able to spring back to its original shape following compression. Unfortunately, cartilage is weak in resisting shear forces (twisting & bending). Because of this weakness, torn cartilage is a common sports injury.

Most LIGAMENTS are cords of DENSE REGULAR CONNECTIVE TISSUE  that attach bone to bone at joints. The ligaments between the vertebrae, however, are made of ELASTIC CONNECTIVE TISSUE.

3) Tendons connect muscles to bone


4) Ligaments connect bone to bone


II. Bone Biology

Like all connective tissue, BONE TISSUE contains a great deal of extracellular matrix. The extracellular matrix of bone consists of 25% water, 50% mineral salts & 25% collagen. The mineral salts include primarily calcium salts, like calcium phosphate and calcium carbonate. There are also small amounts of magnesium and fluoride. The mineral salts give bone its hardness, which allows bone to resist compression. Collagen contributes to the bone's great tensile strength, making the bone more resilient and pliable, and less brittle. 

1) Bone is composed of organic material (mostly collagen, a spongy

    protein), within an inorganic matrix called hydroxyapatite (mostly

    calcium and potassium).


2) Bone tissue consists of three specialized cell types, osteoblasts,

    osteocytes, and osteoclasts


          a. Osteoblasts are bone-forming cells, which line the surface of a

             bone’s structure.

          b. Osteocytes are bone cells and are found within the bone’s


          c. Osteoclasts are cells that resorb bone trough a degradation



3) Process of Bone Formation


At birth, most of the skeletal system is composed of cartilage, which over time is replaced by bone. By the early twenties, most bone growth is complete, although bone is remodeled throughout life.


Your bones are constantly remodeled throughout your life. This process helps to keep them strong and to maintain their integrity for withstanding stresses, and maintaining homeostasis. In a healthy adult the rate of resorption (breakdown) roughly equals the rate of bone deposition. In older people the rate of deposition often falls below that of resorption and osteoporosis results. Bone remodeling is stimulated by physical stress placed on the bone by exercise, and will be tailored to provide specific adaptation to that stress. Thus a weight lifter's bones will show growth patterns peculiar to the stresses placed on the bones.

Through the process of remodeling, osteoclasts circulate throughout the bone and look for old or damaged osteocytes to break down, which are then replaced by osteoblasts which lay down new bone tissue.


This breakdown and buildup occurs throughout the bone, but is most visible at the growth plates of the bone which form at the junction of the epiphysis (bone ends), and the diaphysis (bone shaft). This junction is called the epiphyseal plate, located towards the end of the bone shaft.


Finally, some growth occurs in the periosteum, which is a thin sheaf of tissue that covers the outside of the bone surface. The periosteum also serves as an intake of nutrition and gasses.


III. Bone Anatomy


1) There are approximately 206 bones in the human body


2) They can be classified into four main classes


          a. Long bones: main components of limbs, include the femur,     

             humerus, radius and ulna, tibia and fibula.

          b. Short Bones: include metacarpals of hands and metatarsals of feet

          c. Flat Bones: includes cranial bones, innominates and scapula, offer

             protection and large muscle attachments

          d. Irregular Bones: includes vertebra, carpals (hand) and tarsals

             (feet), many of the cranial bones. These bones are generally

             complex in design and serve specialized purposes


3) The skeleton can also be divided into two parts, the axial

    skeleton, and the appendicular skeleton.


          a. The axial skeleton includes the skull or cranium, the vertebral

             column, and the ribs.

          b. The appendicular skeleton includes the pelvic and pectoral

             girdles, as well as the upper and lower limb bones.


The pectoral girdle includes the scapula and clavicle, and

forms the shoulder

                   The pelvic girdle includes innominate or hip bones.

                   The upper limbs include the humerus, radius, ulna, carpals,

                  metacarpals, and hand phalanges.

                   The lower limbs include the femur, tibia, fibula, patella,

                  tarsals, metatarsals, and foot phalanges.


Tissues found in bones:

osseous tissues - bone tissue proper

Osseous tissue has a matrix containing inorganic salts and organic fibers. The inorganic matrix gives the rigidity and hardness to bone and is composed of a combination of calcium and phosphorus salts called hydroxyapatite. The organic collagen fibers give a bone its tensile strength and resistance to stress.

cortical (compact) bone - made of a dense regular arrangement of osteons (Haversian systems). Compact bone is found in the diaphysis (shaft) of long bones and as the outer layer of all bones.

cancellous (spongy) bone - Consists of trabeculae (a network) of thin, connecting spicules which form a meshwork in the interior of bones. Spaces between the trabeculae contain marrow and blood vessels.

red marrow - myeloid (blood producing) tissue found in the spaces in the spongy bone, produces both red and white blood cells. Red marrow does not increase in proportion to bone growth, and in the adult much of the red marrow changes to yellow (fatty) marrow, especially in the medullary canal.

yellow marrow - Consists mostly of fat cells. It can revert to red marrow under extreme hematopoietic stress, such as in blood loss.

periosteum - a fibrous covering of bones which connects to tendons and ligaments and anchors blood vessels and nerves. The inner layer of the periosteum contains osteoprogenitor cells, derived from mesenchyme cells, these are the cells which divide to become osteoblasts under appropriate stimuli. An extremely strong connection is formed with tendons and ligaments because the collagen fibers from these structures, called Sharpey's fibers, extend at an angle into the bone where they are continuous with collagen fibers in its extracellular matrix.

endosteum - fibrous tissue lining the medullary canal. Often only once cells thick its cells are also osteoprogenitor cells.

medullary canal - central canal of a long bone. It makes the bone lighter and in adults contains yellow marrow.

articular cartilage - hyaline cartilage which forms part of synovial joints.

 Haversian systems, are the units of structure in mature bone. They are tightly arranged running generally parallel to the long axis of the bone. At the center of each is an Haversian canal which carries blood vessels and nerves. Canaliculi (small canals) connect the Haversian canals with lacunae containing the osteocytes. Osteocytes extend processes into the canaliculi and receive nutrients and O2 and get rid of wastes and CO2 by diffusion through the canaliculi. The lacunae and canaliculi form lamellae or layers of two types: concentric lamellae form circular rings around each Haversian canal, and interstitial lamellae, derived from previous osteons, fill in the spaces between existing osteons. 

Hormones important to bone growth and homeostasis:

Growth Hormone (GH) - from the anterior pituitary, this hormone is necessary for normal growth and development of the skeleton. A deficiency (hyposecretion) of GH during childhood produces a dwarf, an excess (hypersecretion) produces a giant. Hypersecretion in adulthood produces acromegaly, a disorder in which the shape of many bones, especially those in the face becomes exaggerated.

The thyroid hormones (e.g. thyroxine) - regulate metabolism of most cells including those in bone.

testosterone - this and other androgens are important for growth in mass and density of bone. Testosterone is present in both males and females in varying amounts.

estrogens - these hormones are important for growth in length of bone and for bone maintenance. They too are present in varying amounts in both sexes.

parathyroid hormone - this hormone exerts the primary control in calcium homeostasis. Calcium is necessary in the blood for many functions and when its level falls parathyroid hormone is secreted. This hormone uses several methods to raise calcium levels in the blood: 1) increased Vitamin D production. Vitamin D is a hormone whose precursor is produced in the skin in response to sunlight and then processed in the liver and kidney to become active Vitamin D3. Vitamin D3 increases calcium absorption in the gut. Without this vitamin calcium is not absorbed to any great degree. 2) increased reabsorption of calcium in the kidney. Much calcium is lost to the urine, so when you need more in the blood this is an important source. 3) resorption of bone. PTH increases osteoclastic activity to release calcium into the blood.

Calcitonin - Normally important only in children, this hormone is secreted by special cells in the thyroid. Its function is to stimulate the uptake of calcium into growing bone and the deposition of bone matrix. It is not produced, nor is it effective therapeutically, in adults.

Osteoporosis, a disorder involving demineralization of bone usually associated with older individuals can be related to several factors:

1) deficiency of dietary calcium

2) reduced estrogen levels common in post-menopausal women. This may be treated with HRT, hormone replacement therapy.

3) reduced activity and exercise, including:

4) reduced weight bearing stress on the bones. This is important in stimulating bone growth and replacement at any age.

Osteoporosis treatment may include calcium formulated with other minerals, hormone replacement therapy, calcitonin, and an exercise program.